Ceramics have been utilized in many fields because of not only their outstanding mechanical properties such as heat resistance and abrasion resistance but also their excellent electric and magnetic properties. Among these, zirconia-based ceramic sheets have a superior oxygen ion conductivity and high toughness in addition to these properties. Thus, they are utilized as solid electrolytes for sensors, such as oxygen sensor and humidity sensor, and further as a solid electrolyte for SOFC.
In the electrolyte sheet for SOFC, in addition to enhancing the membrane strength, it is desired to increase the contact area between the electrolyte sheet and an anode and a cathode provided on the surfaces of the electrolyte sheet to enhance the effective area of electrochemical reaction and power generation performance. Also, in the electrolyte sheet for SOFC, in order to achieve a stable power generation performance, it is desired to prevent the anode and cathode from delaminating from the electrolyte sheet during the operation of the fuel cell or while the temperature is increased or decreased. To achieve such objectives, the inventor focused on the surface roughness of the electrolyte sheet and investigated a wide range of surface roughness. Through this work, the inventor found Patent Documents 1-3 which infer that an electrolyte sheet having a specific surface roughness have a potential to achieve the above objectives.
SOFC generates electricity by supplying two kinds of gases, namely fuel gas and oxidizing gas, respectively to an anode and a cathode, which are separated by the electrolyte sheet, and by performing electrochemical reactions in each electrode at a high temperature such as 600° C.-950° C. Even when the electrolyte sheet has the surface roughness mentioned above, the voltage of a single cell is as low as about 1V. Thus, in the actual operation of the fuel cell, it is necessary to stack plural single cells and connect them in series (stacking) to obtain the sufficient power. In this arrangement, while the adjacent cells are electrically connected, separators made of metal or ceramic are provided between the single cells to properly distribute the fuel gas and oxidizing gas to the anodes and cathodes respectively through a manifold. The separator is also called interconnector.
However, the electrolyte sheets having the surface roughness mentioned above does not satisfy all the requirements such as formability of electrodes on the electrolyte sheet, adhesion between the electrolyte sheet and the electrodes, reaction area of the electrode such as three-phase boundary, power generation performance, prevention of delamination of the electrodes over time, sealability of fuel gas and/or oxidizing gas between the periphery of the electrolyte sheet and the separator, handling strength, thermal shock resistance against heat cycles due to repeated temperature fluctuation between room temperature and 600° C.-950° C. caused by start/stop of SOFC. In other words, the electrolyte sheets having the surface roughness mentioned above do not satisfy some of these requirements.
For example, in the electrolyte sheet having a surface roughness specified in the Patent Document 2, the adherence of the electrolyte sheet to the electrode layers formed on both sides of the sheet, the reaction area of the electrode, and the power generation performance are improved. However, the electrolyte sheet can be cracked from its periphery when the electrodes are formed on the electrolyte sheet by screen printing, and when the electrolyte sheets and separators are alternately stacked. Therefore, careful handling is necessary.